The present invention relates to a method and an apparatus for electrolytic treatment of the surface of metal web in which the stability of the electrodes is improved.
Examples of methods of applying an electrolytic treatment to the surface of a metal member made of aluminum, iron or the like are the plating method, the electrolytic roughening method, the electrolytic etching method, the anodic oxidation method, the electrolytic coloring method, and the electrolytic satin finishing method, all which have been extensively employed in the art. D.C. sources, A.C. sources, superposed-waveform current sources, and thyristor-controlled special-waveform or square-wave A.C. sources have been employed with these methods in order to meet quality requirements of the electrolytic treatment or to improve the reaction efficiency. For instance, U.S. Pat. No. 4,087,341 discloses a process in which an A.C. current is applied in the electrolytic treatment of an aluminum plate with the voltage applied to the anode electrode being higher than the applied to the cathode electrode, whereby aluminum substrates for lithographic printing whose surfaces are satisfactorily electrograined are obtained. When using regulated A.C. current, it is essential to employ electrodes which are highly stable. In general, platinum, tantalum, titanium, iron, lead and graphite are employed as electrode materials. Graphite electrodes are widely employed because they are chemically relatively stable and are of low cost.
FIG. 1 shows an example of a conventional continuous electrolytic treatment system for metal webs which utilizes graphite electrodes. In this system, a metal web 1 is introduced into an electrolytic cell 3 while being guided by a guide roll 2, and is conveyed horizontally through the cell while being supported by a roll. Finally, the web 1 is moved out of the cell passing around a guide roll 4. The electrolytic cell is divided by an insulator 6 into two chambers in which graphite electrodes are arranged on both sides of the metal web 1. A supply of electrolytic solution 9 is stored in a tank 10. A pump 11 supplies the electrolytic solution to electrolytic solution supplying pipes 12 which debouch into the electrolytic cell. The electrolytic solution thus supplied covers the graphite electrodes 7 and 8 and the metal web and then returns to the tank 10 through a discharging pipe 13. A power source 14 connected to the graphite electrodes 7 and 8 applies a voltage thereto. An electrolytic treatment can be continuously applied to the metal web 1 with this system.
The power source 14 may produce (1) direct current, (2) a symmetric alternating current waveform, (3) and (4) an asymmetric alternating current waveform, and (5) and (6) an asymmetric square-wave alternate current waveform as shown in FIG. 2. In general, with the A.C. waveform, the average value of the forward current I.sub.n is not equal to the average value of the reverse current I.sub.r.
A graphite electrode is considerably stable when used a cathode electrode. However, when a graphite electrode is used as an anode electrode, it is consumed in the electrolytic solution, forming CO.sub.2 by anode oxidation and, at the same time, it decays due to erosion of the graphite interlayers, which occurs at a rate depending on electrolytic conditions. When decay occurs, the current distribution in the electrode changes so that the electrolytic treatment becomes nonuniform. Therefore, the occurrence of such a phenomenon should be avoided in a case where the electrolytic treatment must be performed with high accuracy. Accordingly, it is necessary to replace the electrodes periodically. This requirement is a drawback in mass production, and is one of the factors which lowers productivity.
An object of the invention is to provide an electrolytic treatment method in which, based on the properties of graphite, the electrodes are maintained sufficiently stable even in electrolytic treatment using an asymmetric A.C. waveform.